The sentence is not perfectly well written ie. it's using a grammer rule for much/many to make its meaning clear. It's only obvious that 'much' refers to elasticity because it's the singular noun. 'Many' would refer to buyers and sellers because they're plural. The rule is straightforward, despite the overanalysis (and incorrect reasoning) in some of the answers below: 'much' refers to a singular noun, 'many' refers to a plural noun.
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Pete855217Jul 20 '14 at 6:20

The meaning expressed by *how much( is here very different than the one that how many would express.

Here, how much represents the strength of the reaction, NOT the number of people who react to the changes in market conditions. The answer here will be in term of: strongly, not that strongly, etc. if the approach is qualitative.

If how many was used the focus would be on counting the numer of buyers and sellers, and the answer would be: 2 mio, 300, etc.

If one buyer, responds strongly the result will still be strong probably.